cppgc: Fix TraceTrait for JSMember

Template specializations must use exact types to match.

Bug: chromium:1056170
Change-Id: I2278e9b40712fc209044fe565023029cc5ae3ff3
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2474776
Commit-Queue: Michael Lippautz <mlippautz@chromium.org>
Reviewed-by: Omer Katz <omerkatz@chromium.org>
Cr-Commit-Position: refs/heads/master@{#70527}

include/v8-cppgc.h

@@ -66,6 +66,9 @@ class V8_EXPORT JSMemberBase {
   friend class v8::JSMember;
   friend class v8::JSVisitor;
   friend class v8::internal::JSMemberBaseExtractor;
+
+  template <typename U>
+  friend bool operator==(const internal::JSMemberBase&, const Local<U>&);
 };
 
 JSMemberBase& JSMemberBase::CopyImpl(const JSMemberBase& other) {
@@ -93,6 +96,21 @@ void JSMemberBase::Reset() {
   SetSlotThreadSafe(nullptr);
 }
 
+template <typename U>
+inline bool operator==(const v8::internal::JSMemberBase& lhs,
+                       const v8::Local<U>& rhs) {
+  v8::internal::Address* a = reinterpret_cast<v8::internal::Address*>(lhs.val_);
+  v8::internal::Address* b = reinterpret_cast<v8::internal::Address*>(*rhs);
+  if (a == nullptr) return b == nullptr;
+  if (b == nullptr) return false;
+  return *a == *b;
+}
+
+template <typename U>
+inline bool operator!=(const v8::internal::JSMemberBase& lhs, const U& rhs) {
+  return !(lhs == rhs);
+}
+
 }  // namespace internal
 
 /**
@@ -225,9 +243,9 @@ class JSVisitor : public cppgc::Visitor {
 
 namespace cppgc {
 
-template <>
-struct TraceTrait<v8::internal::JSMemberBase> {
-  static void Trace(Visitor* visitor, const v8::internal::JSMemberBase* self) {
+template <typename T>
+struct TraceTrait<v8::JSMember<T>> {
+  static void Trace(Visitor* visitor, const v8::JSMember<T>* self) {
     static_cast<v8::JSVisitor*>(visitor)->Trace(*self);
   }
 };

test/unittests/heap/js-member-unittest.cc

@@ -3,6 +3,7 @@
 // found in the LICENSE file.
 
 #include "include/v8-cppgc.h"
+#include "src/heap/cppgc/visitor.h"
 #include "test/unittests/test-utils.h"
 #include "testing/gtest/include/gtest/gtest.h"
 
@@ -160,5 +161,44 @@ TEST_F(JSMemberTest, EqualityHeterogenous) {
   }
 }
 
+namespace {
+
+// Must be used on stack.
+class JSVisitorForTesting final : public JSVisitor {
+ public:
+  explicit JSVisitorForTesting(v8::Local<v8::Object> expected_object)
+      : JSVisitor(cppgc::internal::VisitorFactory::CreateKey()),
+        expected_object_(expected_object) {}
+
+  void Visit(const internal::JSMemberBase& ref) final {
+    EXPECT_EQ(ref, expected_object_);
+    visit_count_++;
+  }
+
+  size_t visit_count() const { return visit_count_; }
+
+ private:
+  v8::Local<v8::Object> expected_object_;
+  size_t visit_count_ = 0;
+};
+
+}  // namespace
+
+TEST_F(JSMemberTest, JSMemberTrace) {
+  v8::Local<v8::Context> context = v8::Context::New(v8_isolate());
+  v8::Context::Scope context_scope(context);
+  {
+    v8::HandleScope handles(v8_isolate());
+    v8::Local<v8::Object> local =
+        v8::Local<v8::Object>::New(v8_isolate(), v8::Object::New(v8_isolate()));
+    v8::JSMember<v8::Object> js_member(v8_isolate(), local);
+    JSVisitorForTesting visitor(local);
+    // Cast to cppgc::Visitor to ensure that we dispatch through the base
+    // visitor and use traits.
+    static_cast<cppgc::Visitor&>(visitor).Trace(js_member);
+    EXPECT_EQ(1u, visitor.visit_count());
+  }
+}
+
 }  // namespace internal
 }  // namespace v8